1. Technical Field
The field of invention relates to liquid purification, treatment and separation processes, including controlling process in response to a sensed condition.
2. Description of Related Art
Many fluid handling systems require the monitoring of selected characteristics of the fluid stream in order to determine and control the amount of a treatment chemical to be introduced into the fluid stream. For instance, in wastewater treatment facilities a main stream of wastewater typically is treated with a chlorine-containing compound, often chlorine gas or sodium hypochlorite, to oxidize organic compounds in the fluid stream. Upon exposure to sufficient levels of oxidizing chlorine species for a sufficient period of time, the gross levels of bacteria in the wastewater stream are reduced to allowable levels for discharge from the wastewater treatment plant into an existing stream, river, estuary, etc.
It is undesirable to under-treat the wastewater stream because excessive levels of bacteria will thus remain in the wastewater stream. It is also undesirable to over-treat the wastewater with chlorine because an excessively high residual chlorine level in the wastewater stream can harm the natural flora and fauna at and beyond the outfall (i.e., the end of the pipe of the treatment plant, where the effluent meets the natural body of water). Additionally, when a waste stream is over-treated and is properly detected, an additional chemical, such as sulfur dioxide, typically is added to neutralize the excess chlorine prior to discharge at the outfall. This adds to the cost of the excess chlorine or other treatment chemical, and in sum represents an unnecessary operating cost that can be minimized or avoided by implementation of the present invention.
Many advanced wastewater treatment systems include a residual chlorine analyzer downstream from the chlorinator which monitors the levels of chlorine residual remaining in the wastewater stream. Controllers are known which utilize the results of this downstream analyzer to provide a feedback signal to the chlorinator. In essence, if an amount of residual chlorine is too high at the downstream analyzer, the chlorinator receives a feedback signal which decreases the rate of introduction of chlorine. If the analyzer detects that no chlorine or too little chlorine remains, which indicates that the bacteria level has not been sufficiently lowered, the feedback signal will cause the chlorinator to increase the rate with which it introduces chlorine.
While such residual chlorine analyzers and feedback control systems are generally effective, they suffer from numerous drawbacks. For instance, when the analyzer detects a sub-optimal amount of chlorine residual, additional treatment must still occur to the wastewater stream to properly treat the wastewater. The delay associated with the distance between the analyzer and the chlorinator and the amount of time it takes for the chlorine analyzer to detect any change in chlorine residual causes problems in the control. This can result in uneven, and at times, improper treatment of wastewater. That is, such feedback systems have a tendency to enter into an oscillatory state between over and under chlorination which only relatively slowly resolves itself. This situation is especially common when the requirement, or demand, for chemical treatment within the wastewater stream is fluctuating. In wastewater treatment systems, demand typically is expressed as the sum of the Biological Oxygen Demand (“BOD”) and the Chemical Oxygen Demand (“COD”) (collectively, “CBOD”).
Also, wastewater treatment facilities can incur fines for releasing wastewater which is either under-treated or over-treated with chlorine containing compounds. As noted above, wastewater treatment facilities additionally suffer financially from the unnecessary use of excess chlorine and chlorine-neutralizing chemicals, such as sulfur dioxide, when the chlorination system is not operating optimally.
In past years (and to a much lesser extent currently), dosing was based on results of laboratory or bench testing the influent chemical concentration together with measuring its flow. Subsequently, dose calculations were performed and the dosing device, a chemical feed pump for example, was manually adjusted according to the calculations.
In recent years, reliable automatic analyzers for chemical concentration have become available enabling automation of the entire dosing procedure. Thus, the need for manual testing and manual adjusting has been practically eliminated. An additional consequence is that the automatic analyzers can also be set up to detect several important chemicals in water treatment. This makes the dosing procedure useful for other applications such as the addition of sodium carbonate into an aerated biological reactor to control nitrification or the addition of iron or aluminum salts before a clarifier to control phosphorus removal. However, it has been recognized that problems can occur during the automatic dosing of a chemical into the treatment system because of, inter alia, the inaccuracies of measurement of chemical demand present in the system and the variable ratio of chemical to liquid when the liquid flow rate or the demand is variable.
Japanese Patent No. Sho 51-130055 to Tokyo Shibaura Electric Co. relates to an apparatus for control of the feed rate of water purification reagents. The apparatus consists of a source water quality measurement meter for measurement of water quality of the source water intake, a reagent feed device, a ratio setting device that maintains a ratio of the reagent feed rate to the source water intake, a settling water quality measurement meter that measures the water quality of settling water and outputs a signal, and a calculating control device that receives the output signals and sets the flow rate of the reagent and sets the ratio setting device. The apparatus measures water quality factors such as source water turbidity, pH, alkalinity and temperature, not concentration of the reagents.
U.S. Pat. No. 4,435,291 to Matsko (the '291 reference) discloses a system for controlling the dosing of chlorine in a system for chlorinating wastewater. In the '291 reference, the chlorine dosage is controlled by electronic controllers according to a derivative of residual chlorine with respect to chlorine dosage. This is stated to provide an accurate control of chlorine to insure oxidation of ammonia in wastewater. Flow transmitters sense the flow of chlorine, of base, or of sulfur dioxide to their respective tanks.
U.S. Pat. No. 4,544,489 to Campbell et al (the '489 reference) discloses a process and apparatus for the controlled addition of a conditioning polymer material to sewage sludge. The '489 reference employs a computer with a connected viscometer. Based upon the shear stresses measured and input to the computer by the viscometer, the system controls the rate of pumping of the polymer to mix with the sludge.
Other references that describe aspects of the relevant art are U.S. Pat. No. 5,011,613 to Feray and Hubele, U.S. Pat. No. 5,869,342 to Stannard et al., U.S. Pat. No. 6,129,104 to Ellard et al., and U.S. Pat. No. 6,346,198 to Watson and Armstrong, each of which is hereby incornorated by reference. Also of relevance is “Problems Involving in Automating the Waste-Water-Treatment Plant,” by Raymond Kudukis, Ch. 10, pp. 74-78 of INSTRUMENTATION CONTROL AND AUTOMATION FOR WASTE-WATER TREATMENT SYSTEMS, ed. By J. F. Andrews et al. Permagon, N.Y. USA, 1974. This reference states, inter alia, that attaining maximum efficiency of a wastewater treatment plant with automated control systems is expected to remain difficult given the fact that “periodic intensity due to storm flow or periodic lows during dry-weather spells . . . much of the time the flow into the plant is either above or below the maximum efficiency level.” Also of relevance is WASTEWATER DISINFECTION—Manual of Practice FD-10, pp. 144-155, Water Environmental Federation, Alexandria, Va. USA 1996, which teaches a number of standard feed control strategies for introduction of disinfectant to wastewater. None of these standard feed control strategies are directed to dual feeding into a side stream to provide dosing for the main flow, such as is disclosed and claimed herein.
None of the references teach or suggest a method for automatic controlled dosing of a treatment chemical into a flow stream in a liquid treatment system that more correctly measures, over small or “real-time” increments, the amount of chemical required based both on varying flow rate and on varying demand for the treatment chemical. Consequently, there remains a need to improve the accuracy and/or the precision of dosing a liquid flow, such as wastewater, with a desired chemical treatment.